Editing Field electron emission (section)

=== Large-area field emission sources: vacuum nanoelectronics ===

==== Materials aspects ====
Large-area field emission sources have been of interest since the 1970s. In these devices, a high density of individual field emission sites is created on a substrate (originally silicon). This research area became known, first as "vacuum microelectronics", now as "vacuum nanoelectronics".

One of the original two device types, the "[[Spindt tip|Spindt array]]",<ref name=SBHW76>{{cite journal|doi=10.1063/1.322600|title=Physical properties of thin-film field emission cathodes with molybdenum cones|year=1976|last1=Spindt|first1=C. A.|journal=Journal of Applied Physics|volume=47|pages=5248–5263|bibcode = 1976JAP....47.5248S|issue=12 }}</ref> used [[integrated circuit|silicon-integrated-circuit (IC)]] fabrication techniques to make regular arrays in which [[molybdenum]] cones were deposited in small cylindrical voids in an oxide film, with the void covered by a counterelectrode with a central circular aperture. This overall geometry has also been used with [[carbon nanotubes]] grown in the void.

The other original device type was the "Latham emitter".<ref name=la95>{{cite book|editor=R.V. Latham|title=High-Voltage Vacuum Insulation: Basic Concepts and Technological Practice|publisher=Academic, London|year=1995}}</ref><ref name=F01>{{cite journal|doi=10.1016/S0038-1101(00)00208-2|title=Low-macroscopic-field electron emission from carbon films and other electrically nanostructured heterogeneous materials: hypotheses about emission mechanism|year=2001|last1=Forbes|first1=R|journal=Solid-State Electronics|volume=45|pages=779–808|bibcode=2001SSEle..45..779F|issue=6}}</ref> These were MIMIV (metal-insulator-metal-insulator-vacuum) – or, more generally, CDCDV (conductor-dielectric-conductor-dielectric-vacuum) – devices that contained conducting particulates in a dielectric film. The device field-emits because its microstructure/nanostructure has field-enhancing properties. This material had a potential production advantage, in that it could be deposited as an "ink", so IC fabrication techniques were not needed. However, in practice, uniformly reliable devices proved difficult to fabricate.

Research advanced to look for other materials that could be deposited/grown as thin films with suitable field-enhancing properties. In a parallel-plate arrangement, the "macroscopic" field ''F''<sub>M</sub> between the plates is given by {{nowrap|1=''F''<sub>M</sub> = ''V''/''W''}}, where ''W'' is the plate separation and ''V'' is the applied voltage. If a sharp object is created on one plate, then the local field ''F'' at its apex is greater than ''F''<sub>M</sub> and can be related to ''F''<sub>M</sub> by
: <math> F = \gamma F_{\mathrm{M}}.</math>

The parameter ''γ'' is called the "field enhancement factor" and is basically determined by the object's shape. Since field emission characteristics are determined by the local field ''F'', then the higher the ''γ''-value of the object, then the lower the value of ''F''<sub>M</sub> at which significant emission occurs. Hence, for a given value of ''W'', the lower the applied voltage ''V'' at which significant emission occurs.

For a roughly ten year-period from the mid-1990s, there was great interest in field emission from plasma-deposited films of [[diamond-like carbon|amorphous and "diamond-like" carbon]].<ref name=Ro02>{{cite journal|doi=10.1016/S0927-796X(02)00005-0|title=Diamond-like amorphous carbon|year=2002|last1=Robertson|first1=J|journal=Materials Science and Engineering: R: Reports|volume=37|pages=129–281|issue=4–6|s2cid=135487365 }}</ref><ref>{{cite book|author1=S.R.P. Silva |author2=J.D. Carey |author3=R.U.A. Khan |author4=E.G. Gerstner |author5=J.V. Anguita |chapter=9|title=Handbook of Thin Film Materials|editor=H.S. Nalwa|publisher=Academic, London|year=2002}}</ref> However, interest subsequently lessened, partly due to the arrival of [[carbon nanotube|CNT]] emitters, and partly because evidence emerged that the emission sites might be associated with particulate carbon objects created in an unknown way during the [[chemical vapor deposition|deposition process]]: this suggested that [[quality control]] of an industrial-scale production process might be problematic.

The introduction of CNT field emitters,<ref name=JB04/> both in "mat" form and in "grown array" forms, was a significant step forward. Extensive research has been undertaken into both their physical characteristics and possible technological applications.<ref name=Milne/> For field emission, an advantage of CNTs is that, due to their shape, with its high [[aspect ratio]], they are "natural field-enhancing objects".

In recent years there has also been massive growth in interest in the development of other forms of thin-film emitter, both those based on other carbon forms (such as "carbon nanowalls"<ref>{{cite journal | last1 = Hojati-Talemi | first1 = P. | last2 = Simon | first2 = G. | year = 2011| title = Field emission study of graphene nanowalls prepared by microwave-plasma method | journal = Carbon | volume = 49 | issue = 8| pages = 2875–2877 | doi = 10.1016/j.carbon.2011.03.004 | bibcode = 2011Carbo..49.2875H }}</ref>) and on various forms of wide-band-gap semiconductor.<ref name=XH05>{{cite journal|doi=10.1016/j.mser.2004.12.001|title=Novel cold cathode materials and applications|year=2005|last1=Xu|first1=N|last2=Huq|first2=S|journal=Materials Science and Engineering: R: Reports|volume=48|pages=47–189|issue=2–5}}</ref> A particular aim is to develop "high-''γ''" nanostructures with a sufficiently high density of individual emission sites. Thin films of nanotubes in form of nanotube webs are also used for development of field emission electrodes.<ref name=understand >{{cite journal | year = 2013| title = Understanding parameters affecting field emission properties of directly spinnable carbon nanotube webs | journal = Carbon | volume = 57 | pages = 388–394 | doi = 10.1016/j.carbon.2013.01.088 | last1 = Hojati-Talemi | first1 = Pejman | last2 = Hawkins | first2 = Stephen | last3 = Huynh | first3 = Chi | last4 = Simon | first4 = George P. | bibcode = 2013Carbo..57..388H }}</ref><ref name=high >{{cite journal | year = 2013| title = Highly efficient low voltage electron emission from directly spinnable carbon nanotube webs | journal = Carbon | volume = 57 | pages = 169–173 | doi = 10.1016/j.carbon.2013.01.060 | last1 = Hojati-Talemi | first1 = Pejman | last2 = Hawkins | first2 = Stephen C. | last3 = Huynh | first3 = Chi P. | last4 = Simon | first4 = George P. | bibcode = 2013Carbo..57..169H }}</ref><ref>{{cite journal | year = 2010| title = Electron field emission from transparent multiwalled carbon nanotube sheets for inverted field emission displays | journal = Carbon | volume = 48 | pages = 41–46 | doi = 10.1016/j.carbon.2009.08.009 | last1 = Kuznetzov | first1 = Alexander A. | last2 = Lee | first2 = Sergey B. | last3 = Zhang | first3 = Mei | last4 = Baughman | first4 = Ray H. | last5 = Zakhidov | first5 = Anvar A. | issue = 1 | bibcode = 2010Carbo..48...41K }}</ref> It is shown that by fine-tuning the fabrication parameters, these webs can achieve an optimum density of individual emission sites.<ref name=understand/> Double-layered electrodes made by deposition of two layers of these webs with perpendicular alignment towards each other are shown to be able to lower the turn-on electric field (electric field required for achieving an emission current of 10&nbsp;μA/cm<sup>2</sup>) down to 0.3&nbsp;V/μm and provide a stable field emission performance.<ref name=high/>

Common problems with all field-emission devices, particularly those that operate in "industrial vacuum conditions" is that the emission performance can be degraded by the adsorption of gas atoms arriving from elsewhere in the system, and the emitter shape can be in principle be modified deleteriously by a variety of unwanted subsidiary processes, such as bombardment by ions created by the impact of emitted electrons onto gas-phase atoms and/or onto the surface of counter-electrodes. Thus, an important industrial requirement is "robustness in poor vacuum conditions"; this needs to be taken into account in research on new emitter materials.

At the time of writing, the most promising forms of large-area field emission source (certainly in terms of achieved average emission current density) seem to be Spindt arrays and the various forms of source based on CNTs.

==== Applications ====
The development of large-area field emission sources was originally driven by the wish to create new, more efficient, forms of [[flat panel display|electronic information display]]. These are known as "[[field-emission display]]s" or "nano-emissive displays". Although several prototypes have been demonstrated,<ref name=Milne/> the development of such displays into reliable commercial products has been hindered by a variety of industrial production problems not directly related to the source characteristics [En08].

Other proposed applications of large-area field emission sources<ref name=Milne>{{cite journal |author=Milne WI |title=E nano newsletter |date=Sep 2008 |issue=13 |url=http://www.phantomsnet.net/Foundation/Enano_newsletter13.php|display-authors=etal}}</ref> include [[microwave]] generation, space-vehicle neutralization, [[X-ray generation]], and (for array sources) multiple [[electron beam lithography|e-beam lithography]]. There are also recent attempts to develop large-area emitters on flexible substrates, in line with wider trends towards "[[plastic electronics]]".

The development of such applications is the mission of vacuum nanoelectronics. However, field emitters work best in conditions of good ultrahigh vacuum. Their most successful applications to date (FEM, FES and EM guns) have occurred in these conditions. The sad fact remains that field emitters and industrial vacuum conditions do not go well together, and the related problems of reliably ensuring good "vacuum robustness" of field emission sources used in such conditions still await better solutions (probably cleverer materials solutions) than we currently have.